Because external contaminants (such as dust, dirt, water, and mud) making their way into an assembly can damage both a shaft seal and its surroundings (including the shaft itself and the housing bore), seals in dirty environments often feature a secondary sealing lip. Whereas the function of the primary lip is to seal in fluid, the function of this secondary lip is to seal out contaminants. In some particularly dirty and demanding applications, more than one secondary lip may be utilized to help keep the seal environment clean.

A secondary lip (also known as a dirt lip, dust lip, or excluder lip) can take two forms: radial or axial. Though different, radial and axial dirt lips have characteristics in common. Both originate at the heel of the elastomeric member in order to seal on the air side (thus preventing harmful contaminants from reaching the primary lip-shaft interface). Unlike many primary lips, radial and axial dirt lips are non-sprung, meaning they do not incorporate a garter spring into their design.

The chief difference between a radial dirt lip and an axial dirt lip is in orientation relative to the shaft. A radial dirt lip extends radially down toward the shaft (see Figure 188). Depending on the specific seal design, a radial dirt lip may or may not actually contact the shaft. Radial dirt lips find their widest use in low speed applications.

Rather than extending toward the shaft like a radial dirt lip, an axial dirt lip extends axially up away from the shaft (see Figure 189). An axial lip makes contact with a vertical surface, one that is perpendicular to the centerline of the shaft. This surface is typically a wear sleeve, bearing surface, or housing. Studies have shown that axial dirt lips last longer and are thus preferable to radial dirt lips for excluding dust and dirt in high-speed applications. Secondary lips are compared in Table 65. Various levels of contamination are classified in Table 66.

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